The studs and bolts used in a wide variety of machinery application usually have a head and a threaded portion or shank. The head is prone to breaking off due to wear and tear or accident. When a stud bolt breaks off, it leaves, in the threaded bore, a threaded portion which must be removed prior to insertion of a new stud or bolt. It has been the practice to remove the threaded remaining portion by drilling into the broken face of the stud or bolt, then driving or screwing an extractor into the drilled hole to engage the stud or bolt remnant and, then, by turning the extractor to unscrew the remnant from the hole.
In the case of small broken studs or bolts, it is highly desirable that the hole be drilled axially for several reasons. First, drilling an off-center hole may damage the bore threads. Second, an axial hole may prevent extractor breakage. An axial hole allows use of the largest possible drill, without damaging the threads, which, in turn, allows use of the largest possible extractor.
Studs and bolts break off in a variety of positions: far into the bore, in the bore near the orifice, at a threaded portion outside the bore, at a shank portion outside the bore or in the larger bore of an attached captive piece.
The typical rough and slanted broken face of a stud or bolt makes drilling an axial hole difficult and consequently several tools have been shown in the prior art for guiding of a center punch and/or drill into engagement with the broken face of a stud or bolt regardless of the location of the break. Examples of such tools are shown in Dunbar U.S. Pat. No. 5,388,933 and Dunbar U.S. Pat. No. 5,820,314 which show a tool for preparing broken studs and bolts for extraction by screw extractors. This tool acts as a guide for centering a center punch and/or drill on the broken face of a stud or bolt. The tool has a nut or collar and a shaft. The nut has a central bore therethrough having a first portion threaded to mate with the stud or bolt threads and a second portion bored to just slide over the stud threads. The shaft has a cylindrical shaft having a first end and a second end and an axial bore therethrough. The shaft includes a first externally threaded portion extending from the shaft first end of threads similar to those of the broken stud. The axial bore in the shaft provides access for drilling an axial hole in the broken face of the stud or bolt.
The provision of an axial hole in the face of a broken stud or bolt is effective for removing a small stud or bolt on the order of one-half inch or less in diameter. However, tools which provide an axial hole in the face are inadequate for the removal of larger studs and bolts. Drilling a center hole in a large stud or bolt face is generally ineffectual since it is difficult or impossible subsequently to apply enough torque to twist the remnant due to the presence of accumulated rust and corrosion.
A tool for centering a drill or punch on the broken face of a large stud or bolts and for using the tool to break up and remove a large stud or bolt which comprises a collar having a central bore therethrough having a first portion threaded to mate with the stud or bolt threads and a second portion bored to just slide over the stud or bolt threads and an externally threaded cylindrical shaft which is threadably receivable in the collar, said threads being similar (that is, of the same size and thread pitch) to those of the broken stud or bolt, the shaft having extending longitudinally therethrough a plurality of off-center holes. The minimum number of off-center holes is three. However, more such holes, preferably in an approximately symmetrical array, up to about nine off-center holes, are used for the removal of very large stud or bolts having a diameter of 3 inches or more. Preferably said off-center holes are three in number, are displaced from each by 120xc2x0, and are equidistant from the center and the inside of said external threads, said holes being adapted to slidably receive an alignment pin or drill bit.
In another aspect, the invention includes the method of removing a large broken stud or bolt from a bore by placing the collar having a central bore therethrough having a first portion threaded to mate with the stud or bolt threads and a second portion bored to just slide over the stud or bolt threads over a broken bolt having a face protecting above the surrounding surface, said bolt having exposed threads, inserting an externally threaded shaft into said collar, said shaft having a plurality of approximately symmetrically arrayed off-center longitudinal holes therethrough, slidably inserting a drill bit through one of said holes and drilling into the face of said bolt, inserting an alignment pin in the first hole drilled, and then drilling additional holes into the face of the bolt through other of the off-center holes, removing said collar and shaft, enlarging the drilled holes, and inserting a punch into the bolt to break up the remains of the bolt to collapse the remains, and removing the remains.
The invention further includes the method of removing a large broken stud or bolt from a bore by placing the collar having a central bore therethrough having a first portion threaded to mate with the stud or bolt threads and a second portion bored to just slide over the stud or bolt threads over a broken bolt having a face protecting above the surrounding surface, said bolt having an exposed smooth shank, inserting an externally threaded shaft into said collar, said shaft having three longitudinal holes therethrough, slidably inserting a drill bit through one of said holes and drilling into the face of said bolt, inserting an alignment pin in the first hole drilled, and then drilling the second and third holes into the face of the bolt, removing said collar and shaft, enlarging the three drilled holes, and inserting a punch into the bolt to break up the remains of the bolt to collapse the remains, and removing the remains.
The invention still further includes the method of removing a large broken stud or bolt from a bore by placing the collar having a central bore therethrough having a first portion threaded to mate with the stud or bolt threads and a second portion bored to just slide over the stud or bolt threads over a broken bolt having a face recessed below the surrounding surface, inserting an externally threaded shaft into said collar, said shaft having three longitudinal holes therethrough, slidably inserting a drill bit through one of said holes and drilling into the face of said bolt, inserting an alignment pin in the first hole drilled, and then drilling the second and third holes into the face of the bolt, removing said collar and shaft, enlarging the three drilled holes, and inserting a punch into the bolt to break up the remains of the bolt to collapse the remains, and removing the remains.
In another embodiment, the invention includes the method of removing a large broken stud or bolt from a bore by placing the collar having a central bore therethrough having a first portion threaded to mate with the stud or bolt threads and a second portion bored to just slide over the stud or bolt threads over a broken bolt having a face essentially flush with the surrounding surface, inserting an externally threaded shaft into said collar, said shaft having three longitudinal holes therethrough, slidably inserting a drill bit through one of said holes and drilling into the face of said bolt, inserting an alignment pin in the first hole drilled, and then drilling the second and third holes into the face of the bolt, removing said collar and shaft, enlarging the three drilled holes, and inserting a punch into the bolt to break up the remains of the bolt to collapse the remains, and removing the remains.